The invention relates to a sensor for determining the mass flow of a fluid (mass flow sensor), to a pipe provided with a mass flow sensor of this type, as well as to a method for determining the mass flow of a fluid in a pipe.
Conventional mass flow sensors for determining the mass flow of a fluid, meaning a liquid or a gas, on the one hand are based on a use of differential pressure methods which, however, cause a lasting pressure loss in the flow as a result of the measuring operation and, on the other hand, are based on electronic methods which are influenced considerably by strong electromagnetic fields in the area surrounding the sensor, thereby resulting in high measuring errors or in a considerable calibration expenditure. Different types of mass flow sensors have therefore already been proposed which are provided with a fiber Bragg grating sensor (FBG sensor) that permits a precise determination of the change in the length of a glass fiber strand.
Japanese Patent JP 2005003535 A discloses an optical device for determining the direction and/or the speed of a flow, which is based on an element with a thereon mounted glass fiber and an FBG sensor being deformed by the hydraulic pressure. The disadvantage of this arrangement is that a noticeable hydraulic pressure is generated only in a liquid, so that the device is not suitable for use with gas flows. The device furthermore requires a deformable element which is an essential disadvantage, in particular with extreme changes in temperature during which the material characteristics of this element can change considerably, as well as at low temperatures since practically no deformable materials exist, which can be used at these temperatures.
A flow sensor is also known from the document European Patent Application EP 1936332 A1 for which a flow element is introduced into the flow to detect the Kámán vortices with the aid of FBG sensors. This sensor has the disadvantage of being located in the flow, thereby resulting in a higher pressure loss as compared to an unobstructed pipe and/or conduit flow. A further disadvantage is that the sensor detects the flow-characteristic Kámán vortices with the aid of a FBG sensor. As a result, it is impossible to determine whether the length change in the flow meter occurs as a result of the change in the flow resistance or only as a result of a temperature change.
Japanese Patent JP 2007017337 A describes a device for determining flow speed on the basis of measuring a dynamic pressure that is exerted onto a wall element, wherein the FBG sensor is located inside an airtight chamber. The disadvantage of this arrangement is that a flow must be directed toward the measuring arrangement, so that it is not suitable for measuring an inside flow and thus is not suitable without problems as a mass flow sensor. The installation inside a pipe would result in considerable pressure loss. Since the sensor is located inside an air-tight chamber, the temperature correction only relates to the temperature-dependent length expansion or elongation of the glass fiber.
British Patent GB 2454613 A discloses a glass fiber with at least one FBG sensor which is inserted into the flow. To increase the signal strength, one or several flow-shaped elements, in particular spheres or ellipsoids, are fixedly connected to the glass fiber. When using several elements to reinforce the mechanical load on the glass fiber, the device represents a type of pearl necklace which is inserted into the flow. The disadvantage of this arrangement is that the reinforced glass fiber must be inserted into the flow, thereby causing an additional pressure loss. In contrast to a liquid flow, a gas flow requires an increase in the number of flow-shaped elements and in the size of the elements, thereby further increasing the pressure loss, wherein it is an additional disadvantage that no distinction can again be made to determine whether the glass fiber expansion is due to the change in the flow resistance or to a temperature change.
U.S. Pat. No. 6,408,698 B1 discloses an electronic sensor which is inserted into the wall and detects the forces resulting from the flow via expandable connections, using an electronic sensor. This arrangement has the disadvantage that the signal changes as a result of electromagnetic fields, wherein this requires a calibration of the sensor in the magnetic field with respect to size and orientation of the sensor relative to the magnetic field. Each individual sensor in this case must be calibrated for its respective use in order to take into consideration production tolerances. In particular when using the sensors in the presence of cryogenic temperatures having magnetic fields impressed from the outside, the calibration expenditure is considerable.
US Patent Publication No. 2009/0133505 A1 describes a device for which the wall shearing stress causes the bending of a rod which, in turn, compresses a micro-resonator. The change in the wave length caused by the mechanical stress exerted on the micro-resonator is then measured. The bending of the rod as well as the mechanical stress and thus the measuring signal depend on the temperature-dependent material values of the rod and the micro-resonator. A disadvantage in this case is that the mechanical stress on the rod depends on the temperature as well as on the purity and structure of the material: impurities or occlusions as well as voids influence the ductility of the rod. The same is correspondingly also true for the micro-sensor, so that each mass flow sensor represents a unique device to be calibrated for temperature and load. Furthermore, owing to the fact that micro-resonators are not sufficiently reproducible, as well as the production of the technically demanding connection between the micro-resonator and the glass fiber, each sensor is a unique device to be calibrated separately. The size of the micro-resonator is furthermore critical with respect to the signal quality. Finally, the bending of the rod not only results in mechanical stress exerted on the micro-sensor, but also leads to a displacement in flow direction, thereby considerably influencing the signal. The reversibility of the movement and taking this into consideration within the framework of a calibration are therefore not necessarily ensured, wherein this also detrimentally affects the permanent functionality of the sensor.
U.S. Pat. Nos. 7,168,311 B2 and 6,426,796 B1 respectively disclose a sensor installed in a wall for detecting forces resulting from a flow via the bending of a rod and with the aid of an optical sensor and interferometry. The disadvantage of these arrangements is that the mechanical stress for bending the rod again depends on the temperature as well as on the purity and structure of the material. The interferometry measurement is based on the beam being transmitted to a plate and that the reflected beam interferes with the transmitted beam, wherein the plate is connected to the rod which is bent as a result of the wall shearing stress. Owing to the bending of the rod, the plate which absorbs the wall shearing stress does not move parallel to the flow direction, but is positioned transverse thereto. A beam impinging on the underside of this plate in that case is no longer reflected normally (180°), but at an angle that deviates from 180°. Since this behavior can be ignored only within an extremely small angular region, it considerably reduces the area of application for the device in flows. Configuring the device in this way furthermore poses maximum requirements with respect to production tolerances.